High frequency energy interchange apparatus



F. B. FANK 2,936,397

HIGH FREQUENCY ENERGY INTERCHANGE APPARATUS May 10, 1960 FREDERICK B. FANK INVENTOR. BY%

Filed Jan. 2, 1959 ATTOEIVE Y United States Patent 6 HIGH FREQUENCY ENERGY INTERCHANG APPARATUS Frederick Beringer Fank, Palo Alto, Calif., assignor to General Electric Company, a corporation of New York Application January 2, 1959, Serial No. 784,661

- 6 Claims. (Cl. 315-35) This invention relates to the class of devices which depends upon an interchange of energy between a stream of electrons and a radio frequency field to provide ampli' fication and/or oscillations. More particularly, the invention relates to the class of highfrequency energy inter! change devices known as traveling-wave tubes which in clude an electron gun for producing a stream of electrons in an interaction region and a radio frequency circuit or transmission line for producing radio frequency fields in the region of interaction. Further, the invention relates to such tubes which are enclosed in a metal ceramic envelope for the purpose of making the construction as rugged as possible and has for one of its principal objects the provision of improved means for transferring high frequency energy between the radio frequency circuit inside the metal-ceramic envelope and a waveguide or transmission line or the like.

A traveling-wave tube typically includes an envelope for enclosing the elements of the tube, an electron gun enclosed within one end of the envelope for producing a stream of electrons in an interaction region and a radio frequency circuit for producing the required radio frequency field in the region of interaction. In addition to these elements, it is generally necessary to provide some means of focusing the stream of electrons throughout its travel in the interaction region, and also means to introduce radio frequency electromagnetic waves into the interaction region and extract electromagnetic waves therefrom. The focusing means usually takes the form of a solenoid or permanent magnet which surrounds the entire traveling-wave tube envelope and produces a magnetic field along the axis of the tube. Such a field tends to confine (focus) the electron stream.

For applications where it is desirable to make a very rugged traveling-wave tube, metal or metal and ceramic envelopes are used. While envelopes of a metal, such as stainless steel, are very rugged, the use of such a material introduces some special problems. For example, the problem of transferring radio frequency electromagnetic waves between external guides and the transmission line inside the metal tube envelope is somewhat unique. In order to provide for such a transfer of radio frequency energy between an external guide and the internal trans mission line, input and output waveguides are typically made a part of the tube envelope. That is, such guides are typically permanently attached to the metallic tube envelope. Such an arrangement requires that a radio frequency transparent window be placed within the guide so that the entire tube capsule, including the input and output waveguide sections, may be evacuated. These windows generally consist of a thin piece of ceramic across the waveguide or a larger piece disposed across the waveguide with appropriate steps for preventing reflections at the radio frequency windows due to discontinuities between the radio frequency transmission line inside the traveling-wave tube and the input and output waveguide sections.

Although this arrangement is satisfactory, it introduces some obvious disadvantages. For example, the input and output waveguides which are a unitary part of the traveling-wave tube envelope protrude from the barrel of the tube making it almost imperative to make the focusing magnet assembly (either a permanent magnet assembly or a solenoid), a premanent part of the package which includes the tube. It is much more desirable to have the tube designedin such a manner that it is easily extracted from and inserted in its focusing magnet assembly. Thus, the tube can be replaced without disturbing the focusing magnet assembly when tube replacement becomes necessary. In order to accomplish this, the tube should have a slim barrel with no protruding stems or artifices. Another disadvantage of having the unitary tube envelope and input and output waveguide sections is that the matching between the transmission lines must be almost entirely accomplished before the tube is assembled, whereas if the tube and the input and output waveguides are separate items, tuning or matching may be done in the separate input and output waveguide sections which are not necessarily evacuated.

A solution to these problems is described and claimed in the co-pending patent application Serial Number 782,432, filed in the name of Fred M. Schumacher the same day as the present application, and assigned to the assignee of the present invention. The present invention represents an improvement over the structure described and claimed in the co-pending application of Fred M. Schumacher.

In the solution to the problems described in the Schumacher application supra, a window which is substantially transparent to radio frequency electromagnetic waves, preferably a ceramic, is inserted between tubular metal portions of a traveling-wave envelope in order to allow radio frequency energy to be coupled into and out of the envelope. The hollow cylindrical radio frequency windows constitute a portion or section of the evacuated traveling-wave tube envelope. While this solution has generally proved .to be highly satisfactory, the ruggedness of the tube is limited to some extent by the tubular ceramic windows which form a portion of the tube envelope and by the brazes between these windows and the metal body of the tube.

Accordingly, it is an object of the present invention to provide a tube envelope for such traveling-wave tubes which envelope is of a metal ceramic construction, presents a slim barrel-like construction with no protruding stems or artifices, and has a barrel with metal extending over its full length.

In carrying out the present invention, a window which is substantially transparent to radio frequency electromagnetic waves, preferably a ceramic, is inserted between metal portions of a traveling-wave tube envelope in order to allow radio frequency energy to be coupled onto and out of the tube envelope. The radio frequency windows constitute a portion of the evacuated traveling-wave tube envelope but allow metal to be utilized over the entire length of the traveling-wave tube.

The novel features which are believed to be characteristic of this invention are specifically set forth in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which;

Figure 1 is a central vertical longitudinal section through a focusing assembly (including input and output waveguides) showing a side elevation of a traveling-wave tube envelope in place in the focusing assembly;

Figure 2 is an enlarged central vertical longitudinal section which is broken through the traveling-wave tube illustrated in Figure 1;

.at its opposite ends to the .two metal tubes.

Figure 3 is a transverse section through the tubular window or body portion of the traveling-wave tube of Figure 2 taken on section line 3-3 of that figure;

Figure 4 is a transverse section through a tubular window or body portion of a traveling-wave tube similar to the section of Figure 3 but illustrating another embodiment of the present invention;

Figure 5 is an enlarged segment of a tube body including a radio frequency window and illustrating another embodiment of the present invention; and

Figure 6 is a transverse section through the window and tube body of Figure 5 taken on section ilne fi-6 of that figure.

Figure 1 illustrates the metal ceramic traveling-wave tube envelope 10 inserted in its focusing magnet assembly 11. The figure does not show a complete magnet assembly, since the assembly does not constitute a part of the present invention. However, the magnet assembly Elia. illustrated includes a section of a solenoid which provides focusing for the electron stream within the vacuum tube, has the configuration of a right circular cylinder, and surrounds the entire envelope 1%. Input and output wavethe opposite end of the tube from the electron gun enclosing portion 15, is another tubular metal section 23 which is coaxial with respect to the rest of the barrel of the electron tube and spaced from the transmission line enclosing section 20 by an output tubular ceramic window 25. The metal section 23 at the output end of the tube is called the collector section since it collects residual energy from the electron stream. The collector section 23 is sealed at its outer end with an end plug 24 inserted snugly inside the collecor tube and held therein by a braze. The tubular output window 25 is coaxially positioned with respect to the rest of the barrel and sealed v between the central slow-wave transmission line enclosing guides 13 and 14, respectively, protrude through apertures in the solenoid 12 and constitute a part of the magnet assembly. Since the particular traveling-wave tube illustrated is a forward wave amplifier tube, the input waveguide 13 is brought in near the enlarged end 15 of the traveling-wave tube which encloses an electron gun 17 (best seen in Figure 2), and the output waveguide 14 is brought in near the opposite end of the tube envelope 10. The particular guides 13 and 14 are conventional waveguides with rectangular cross sections and ridges 16 for matching purposes disposed longitudinally along one internal broad wall of the guide. This arrangement is Well known in the art and therefore is not illustrated or described in detail.

The portion of the tube envelope 10 which extends outwardly from the enlarged electron gun enclosing end 15 is a tubular or barrel-like member made up of sections of metal tubing and tubular sections of ceramic which form the input and output windows. Referring specifi cally to Figures 1 and 2, it is seen that a tubular or cylindrical metallic member 18, which has an outwardly extending disc-shaped flange 19, extends out of and coaxially with the front end of the cylindrical enlarged gun enclosing end 15 of the envelope 10. Another cylindrical or tubular metal portion 20 is positioned coaxially with respect to the input section 18 and is spaced therefrom by a tubular ceramic window 21 which is coaxial with and positioned between the two tubular metal sections 18 and 20. A braze is made between the ends of the ceramic window 21 and the adjoining ends of the metal tubes 18 and 20.

In order to reinforce the window section of the slender barrel-like member, a metal reinforcing tab 9, which is long enough to bridge the entire window 21 and overlap the ends of both the tubular sections 18 and 20, is placed across one section of the window and brazed The configuration of the input window 21 and the reinforcing tab 9 may best be seen from the cross sectional view of Fig- 'ure 3. From this view it is seen that the metal tab 9 generally has the shape of a segment of a right circular cylinder having a diameter such that its internal periphery fits around the external periphery of the two metal tubular portions 18 and 20 of the barrel of the electron tube 10. The tab 9 thus provided structurally reinforces the barrel of the tube 10 and makes it stronger and more rugged. At the same time, it is extremely useful in providing a radio frequency match between an external waveguide and the waveguide inside the tube barrel.

The long tubular portion 2% of the tube barrel constitutes an enclosure for a helical slow-wave transmission line 2-2 which transmits the radio frequency energy down the length of the central portion 20 ofthe tube barrel. At

portion 29 and the metal collector portion 23 to couple radio frequency energy out of the output end of the traveling-wave tube 10.

The tubular ceramic output window 25 has the same general configuration as the input window 21 and, like the input window 21, it is brazed between two metallic tubular portions of the barrel, i.e., transmission line en.- closing portion 2t and collector portion 23. Further, like the input window 21, this joint is provided with a metal reinforcing tab'8 which also has the shape of a segment of a right circular cylinder and is long enough to bridge the output window 25. Thus the metal rein: forcing tab 8 is placed across the ceramic output window 25 with its ends overlapping the metal tube portions 20 and 25 on opposite sides of the window and is brazed in place to form a strong rugged connection between the metal end portions of the tube.

Obviously, the reinforcing tabs 8 and 9 for the windows 25 and 21, respectively, can be provided in a number 1 of different ways and still be within the scope of the present invention. The important thing is that the reinforcing portions be provided in such a manner that the barrel of the tube has at least a portion which is continuously metal over its full length. In this manner, the strength and ruggedness of a single long metal'tube are provided while providing for radio frequency electromagnetic Waves to be coupled into and out of the tube barrel. Figures 4, 5 and 6 show examples of some other embodiments or arrangements.

In Figure 4 a cross section of the tubular ceramic input window 23. is shown looking toward the collector end of the tube so that the metal tube 20 which encloses the transmission line 22 is seen. The metal reinforcement illustrated in this figure constitutes a pair of reinforcing tabs 50 and 51 which have the same general shape as the reinforcing tabs 8 and of Figure 2 but are posi-.. tioned on opposite sides of the tube. Since two of these tabs are used, it is necessary that the segment of a cylinder which they define be smaller, i.e., encompass a smaller arc. The segments 50 and 51 bridge the ceramic window 21 and are brazed to the metal tubular portions 18 and 20 at opposite ends of the ceramic window 21 in the same manner as described with respect to the single reinforcing tab 9 of Figures 2 and 3.

The embodiment of Figures 5 and 6 is also similar to the reinforcing tab 9 as illustrated in Figures 2 and 3, but in this instance, the reinforcing tab is made up of: a projection on the transmission line enclosing metal tube 20. A segment of the end of the central transmission line enclosing tubular portion 20 is cut away to form the reinforcing tab 49 which extends toward the collector end of the tube and has the configuration of a segment of a cylinder. The ceramic output window 25 is positioned inside the tab 49 and brazed inside a recessed portion 52 in the tube 20.

The collector tube portion 23 is provided with-asmall recess 53 in one end which fits around the outer periphery of the-output window 25. The collector tube-portion 23 is positioned around the end of the output window 25 in such a manner that it butts against-the tab extension 4% from the central tubular portion 20. The barrel is then made a single rugged unitby forming brazes between the output window 25 and the metal surrounding it and also between the reinforcing tab 49 and the collector tube portion 23.

The construction of any of the embodiments thus far described provides a slender vacuum tight tubular barrel without projections or protuberances which, as seen in Figure 1, is inserted into the magnet assembly 11 in such a manner that it is centrally located in the internal aperture therethrough with the input ceramic window 21 positioned within the input waveguide 13 and the output ceramic window 25 positioned in the output waveguide 14 at the opposite end of the assembly. The reinforcing tabs over the input and output windows provide what is essentially a continuous metal barrel portion while allowing ceramic windows to be placed in the barrel to provide electromagnetic waves to be introduced and extracted from the barrel portion. Further, the shape of the reinforcing tabs may be changed slightly to enhance the radio frequency matching of the slow-wave circuit 22 inside the tube to the waveguides 13 and 14 outside the tube. The added metal reinforcing tabs across the window may be used either as part of the waveguide wall or part of an inductive iris, or as the shorting plane which is often required across the back of the window to help tune out the reactance produced by the ceramic windows in the waveguide.

The means provided for securing the helical slow-wave transmission line 22 in its coaxial position inside the central tubular portion 20 may best be seen in Figure 3. From this figure, it is seen that three dielectric rods 26, sapphire for example, extend down the length of the helical transmission line 22, are spaced equidistance around the external periphery thereof, and are preferably brazed to the individual turns of thehelix 22. This assembly is positioned inside the central tubular portion 1 20 of the tube barrel and the sapphire rods are fixed to the internal periphery of the central metal tube portion 20 as by brazing.

In order to couple electromagnetic waves between the input waveguide 13 and the input end of the slow-wave transmission line 22, a loop 27 is provided at the input end of the line, which loop extends across the ceramic window and is connected to the input metallic tubular portion 18 by some means such as brazing. In a like manner, the output end of the slow-wave transmission line 22 is provided with a coupling loop 28 which extends into the hollow interior of the output window 25 adjacent one wall, doubles back upon itself, and is fixed to the internal surface of the central barrel portion 20 in order to provide coupling of electromagnetic waves between the slow-wave circuit and the output waveguide 14. Thus, radio frequency electromagnetic waves are introduced in the input waveguide, coupled onto the slowwave transmission line 22 inside the central barrel portion 20 of the traveling-wave tube, propagated down an interaction region internally of the slow-wave transmission line 22, and coupled out into the output waveguide 14. As previously indicated, matching is accomplished at the input and output windows by ridges 16 positioned internally of both the input and output waveguides 13 and 14, respectively, in a conventional manner and also by contouring or shaping the reinforcing tabs which bridge the windows.

The electron gun 15 provides the unidirectional (D.C.) energy which is extracted by the electromagnetic wave and converted to radio frequency energy to provide amplification and is illustrated in detail in Figure 2. The particular electron gun illustrated is of a stacked metal ceramic construction and is one of a kind generally referred to as a velocity-jump gun which finds its principal use in low noise traveling-wave amplifiers. The term stacked metal ceramic electron gun refers to the fact that the electrodes for the gun are metal discs stacked between ceramic rings. That is to say, the wall part of the tube housing whichencloses the electron gun is made up'of a series of disc-shaped electrodes enclosing barrel portion by brazing a disc-shaped flange 19 which extends outwardly from the input tubular section 18 between two of the ring-shaped ceramic spacers 41. The opposite end of the envelope is closed by providing a tubular portion on the metallic disc-shaped flange 43, at the opposite end of the tube, which tubular portion extends away from the gun in the opposite direction from the barrel. This metallic cylinder 47 may be termed the end closing cylinder. The vacuum tight back end seal for the electron tube is provided by a ceramic cylinder member having one end large enough to fit snugly inside the internal periphery of the tubular end sealing cylinder 47 and a metal tubulation 51 which is sealed to the ceramic cylinder 51 and closed off at its one end after the entire tube is evacuated. it may be noted that the brazes at each end of the stacked gun assembly, that is, the brazes which hold the flange 19 of the input tubular section, and the flange or disc member 49 of the end closing section at the opposite end of the gun enclosing portion, are made by stacking a ceramic insulating and spacer ring 41 on each side of the flanges before the braze is made,

The gun illustrated includes a small cylindrical cathode button member 30 located on the central axis of the electron tube for the purpose of emitting electrons which are formed into a stream and directed longitudinally down the envelope 10. A helically wound filament or heater 31 is provided in back of the cathode member 30 to supply the heat necessary to cause the cathode 3% to emit. The cathode member 30 consists of a small end button portion 58 and a larger cylindrical member 59 of the same material directly in back of the small button portion 58. The cathode member 30 is held in this position by a right circular cylindrical sleeve 32 which fits around the outside of the enlarged cylindrical portion 59, extends around the heater 31 and out of the back of a coaxially positioned insulating tubular ceramic sleeve 33. The sleeve 32 is provided with a back end flange portion 34 which is bent around the back end portion of the ceramic sleeve 32 and brazed thereto. The ceramic supporting sleeve 33 is held coaxially within the tube envelope 10 by brazing the front end thereof to the back side of the thin disc-shaped metallic member 36 which is supported coaxially in the envelope wall. This supporting disc 36 is provided with a centrally located aperture 35 through which the small cylindrical cathode button 30 protrudes.

In practice, a metallic tab or car is provided on the outer periphery of each of the disc-shaped gun electrodes so that electrical connections more easily may be made. The tabs are not illustrated in order to simplify the description and the drawings. However, the purpose of each of the electrodes is discussed to provide an understanding of the gun operation. When considering the electrodes in order from the tubulation end of the gun toward the collector end, the first two electrodes it and 42 provide the means of energizing the heater 39. The electrical connection may be traced from the first electrode 49, through a conductor 48, the filament 31, and a conductor 43 to the second electrode '42. The conductor 43 is also connected to the flange 34 on the cathode supporting cylindrical sleeve 32, and, therefore, the second electrode. 42 is also used to establish the potential of the cathode 30. q

The next four gun electrodes 36, 3'8, 39 and 40, respec- -tively; are utilized to shape the potential gradients within the region of the cathode to focus electrons emitted by the cathode and direct them down the longitudinal axis of the tube. Each of these electrodes is provided with a centrally located aperture and the apertures are progressively larger. The electrode 36 which essentially surrounds the button end 58 of the cathode member 30 is made thicker by the addition of a centrally apertured metal disc 37 on the face of the electrode 36 which surrounds the cathode button 30. The exact configuration of the potential shaping electrodes 36 through 45, inclusive, and the potentials established thereon belong to the art of gun design. Therefore, these details are not discussed in detail. However, the potentials established on the electrodes are made progressively higher to provide desired electron accelerating potentials in the gun area.

Since the tubular windows 23. and '25 are sealed and brazed in between the metallic tubular portions 1%, 2i)

and 23 to form a unitary part of the tube envelope, and since metallic reinforcing tabs are provided which bridge the Windows and connect each of the metallic tubular portions 18, 20 and 23, the barrel portion of the tube presents an extremely rugged and strong element while at the same time giving a clean barrel-like appearance with no protruding stems or other artifices. Thus, the objects of the invention are accomplished by providing a traveling-Wave tube which is easily replaceable, extremely rugged, and allows all of the radio frequency matching between internal and external waveguides to be done outside of the vacuum tube.

While particular embodiments of the invention have been illustrated, it will, of course, be understood that the invention is not limited thereto, since many modifications thereto may be made. It is contemplated that the appended claims will cover any such modifications as fall within the spirit and scope of the invention.

What I claim is new and desire to secured by Letters Patent of the United States is:

1. An envelope for a traveling-wave tube including an electron gun enclosing section and an elongated tubular transmission line enclosing barrel section coaxially disposed and sealed in vacuum tight relationship, said transmission line enclosing section comprising alternate portions of metal and ceramic tubing and metal reinforcing tabs bridging the portions of ceramic-tubing between metal tubing portions, said metal reinforcing tabs connected to portions of metal tubing on opposite sides of the said ceramic tubing portions and covering a segment of said ceramic tubing portion thereby to provide metal throughout the length of said elongated tubular transmission line enclosing barrel section and provide an exposed portion of each said ceramic tubing portion for transmission of electromagnetic waves therethrough.

2. An electron tube of the traveling-wave type including an elongated transmission line for propagating electromagnetic waves, electron gun means for projecting a stream of electrons in energy coupling relation with said transmission line, and a vacuum tight envelope means for enclosing and supporting said electron gun means and said transmission line, said envelope including a gun enclosing section and a transmission line enclosing section coaxially disposed and sealed in vacuum tight relationship, said transmission line section comprising alternate portions of metal and ceramic tubing with metal reinforcing tabs bridging each ceramic portion and rigidly sealed to metal portions, said reinforcing tabs covering a peripheral segment of each said ceramic portion on opposite sides of said ceramic tubing portions whereby said transmission line enclosing section is provided with metal throughout its length and ceramic window portions for a the introduction and extraction of electromagnetic waves.

3. In combination in a high frequency energy interchange device of the traveling-Wave type, an elongated transmission line for propagating electromagnetic waves, electron-gun means for projecting a stream of electrons in energy-coupling relation with said transmission line,

and a vacuum tight envelope for enclosing and supporting said stream projecting electron gun means and said transmission line, said vacuum tight envelope including a gun enclosing section and a tubular transmission line section coaxially disposed and sealed in vacuum tight relationship; said transmission line enclosing section comprising alternate portions of metal and ceramic tubing positioned end to end, metal reinforcing tabs covering a peripheral segment of said ceramic tubing portions and bridging each ceramic tubing portion and fixed to metal tubing portions on opposite sides of said ceramic tubing whereby said tubular transmission line enclosing section is provided with metal throughout its length and exposed segments of ceramic tubing for transmission of electromagetic waves therethrough and means for introducing electromagnetic waves on said enclosed transmission line and extracting such waves therefrom.

4. A vacuum tight envelope for a high frequency energy interchange device of the type wherein an interchange of energy takes place between electromagnetic waves propagated down an elongated transmission line and a stream of electrons directed down the length of said elongated transmission line by an electron gun, said vacuum tight envelope including an electron gun enclosing section and a slender tubular transmission line enclosing barrel section coaxially disposed and sealed in vacuum tight relationship, said transmission line enclosing section comprising a tubular metallic input portion, a tubular metallic transmission line enclosing portion, a tubular metallic electron collector portion, and a pair of individual tubular ceramic window portions, said tubular transmission line enclosing portion being coaxially positioned with respect to said tubular input and collector portions, spaced therefrom and sealed thereto by said individual tubular ceramic window portions whereby electromagnetic waves may be introduced on the transmission line and extracted therefrom, and individual metal reinforcing tabs bridging said individual ceramic tubular window portions and fixed to the tubular metallic portions on opposite sides of each window portion to provide mechanical reinforcement for said tubular transmission line enclosing section, said metal reinforcing tabs covering a segment of individual ceramic tubular walls portions to provide for transmission of electromagnetic waves therethrough.

5. In a high frequency energy interchange device of the traveling-wave type, the combination of an elongated transmission line for propagating electromagnetic waves, electron gun means for projecting a stream of electrons in energy coupling relation with said transmission line, and a vacuum tight envelope for enclosing and supporting said stream projecting gun and said transmission line, said envelope including a gun enclosing section and an elongated tubular transmission line enclosing section, said tubular transmission line enclosing section including a tubular metal input portion, a tubular input window and a tubular output window, each of said windows being of a material substantially transparent to electromagnetic waves, a metal tubular transmission line enclosing portion enclosing said transmission line and a tubular metal electron collector portion sealed at one end, said input and output windows being positioned coaxially and sealed between the transmission line enclosing portion and the input and collector points, respectively, to provide for the introduction and extraction of electromagnetic waves, and metal reinforcing tabs defining substantially a segment of tubing similar in cross section to that of said transmission line enclosing section bridging the input and output windows and sealed to the tubular metal portions on each side of each of said windows thereby to provide reinforcement for said elongated tubular transmission line enclosing section along said walls while providing for transmission of electromagnetic waves therethrough.

6. 'In combination in ahigh "frequency energy interchange device of the traveling-wave type, an elongated transmission line for propagating electromagnetic waves, electron gun means for projecting a stream of electrons in energy coupling relation with said transmission line, and a vacuum tight envelope for enclosing and supporting said stream projecting electron gun means and said transmission line, said vacuum tight envelope including a gun enclosing section and a tubular transmission line section coaxially disposed and sealed in vacuum tight relationship; said transmission line enclosing section comprising at least two portions of metal tubing coaxially positioned and at least one window portion of ceramic tubing coaxially positioned between said portions of metal tubing,

metal reinforcing tabs bridging said ceramic tubing portion and fixed to said metal tubing portions on opposite sides of said ceramic tubing and extending around a segment of said ceramic tubing portion whereby said tubular transmission line enclosing section is provided with metal throughout its length and an exposed segment of said ceramic portion.

References Cited in the file of this patent UNITED STATES PATENTS 2,782,339 Nergaard Feb. 19, 1957 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2 936 397 Frederick Beringer Fank May 10, 19 0 7 It is hereby certified that error appears in the printed specification I of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

line 6 for "premanent" read permanent Column 2, column 3, line 12 for "ilne" read line column 7, line; 37 for secured'f read secure --5 column 8 line 65 for 'fpoi-nts' read portions Signed and sealed this 11th day of October 1960.

(SEAL) Attest: KARL H. AXLINE ROBERT C. WATSON Commissioner of Patents Attesting Officer 

